CPAP is a well known treatment for the temporary relief of conditions including obstructive sleep apnea (OSA) and snoring. By this technique, air (or breathable gas) at a pressure elevated above atmospheric pressure is continuously supplied to the entrance of a patient's airway (by the nasal and/or oral route) by means of known arrangements of masks or nasal prongs. The elevated air pressure acts as a pneumatic splint of the patient's airway in the vicinity of the oro- and hypo-pharynx, reducing or eliminating the occurrences of apneas or hypopneas during sleep. A bi-level CPAP device, as opposed to a constant treatment level CPAP device, delivers two distinct pressures during the patient's respiratory cycle--a relatively lower pressure during exhalation and a relatively higher pressure during inhalation. In another form, an automatically adjusting CPAP device may operate to provide a relatively low background pressure which increases to a therapeutic pressure on a needs basis, and preferably at a time to prevent the onset of an apnea.
The term "CPAP" used herein thus is to be understood as including constant, bi-level or adjusting forms of continuous positive airway pressure.
Common to all forms of CPAP apparatus is a nose, mouth or face mask which is fitted to a patient and connected to a flow generator via a flexible air delivery tube/conduit. The flow generator includes an electric motor driving a turbine to provide a supply of air or breathable gas for the administration of CPAP treatment during sleep. The range of positive air pressures supplied at the entrance to a patient's airway typically is in the range 2-20 cm H.sub.2 O. In the pressure regulation control of the flow generator it is usual to have a continuous measure of mask or flow generator delivery pressure, commonly achieved by locating a pressure sensing port at the mask or proximate the flow generator outlet.
In the clinical assessment of the severity of a patient's OSA or upper airway syndrome condition, it is desired to identify the minimum possible CPAP treatment pressure that will alleviate the occurrence of partial or complete apneas during sleep. This is for the reason that the patient is required to expend respiratory effort in expiration against the positive airway pressure, hence it is preferable to minimise the work that must be done to ensure quality of sleep, and as follows administer only the minimal necessary CPAP treatment pressure. In this regard, it is important that the pressure transducer being used to measure the CPAP treatment pressure in control of the flow generator has satisfactory electromechanical characteristics so that the set-point CPAP treatment pressure does not vary significantly. It is known that a reduction of CPAP treatment pressure of as little as 1 cm H.sub.2 O can nullify the therapeutic effect and result in a patient experiencing apneas during sleep.
There is, not unexpectantly, a direct correlation between the electro-mechanical performance of pressure transducers and price, hence the need for accurate pressure measurement is antagonistic towards the need to be able to manufacture CPAP apparatus at a cost that is acceptable to the marketplace. Commercially available pressure transducers, that are not extraordinarily expensive, operate in a small part of their pressure dynamic range in CPAP applications, meaning that there can be a 5-10% drift in the measured value with time due wholly to a pressure transducer operating in a `stretched` region of operation. Such a variation translates to a variation in CPAP treatment pressure of about 1-2 cm H.sub.2 O. There further is market pressure for CPAP treatment to be determined to within an accuracy as low as 0.1 cm H.sub.2 O.
It is therefore one preferred object of the invention to be able to avoid the need to incorporate expensive pressure transducers in CPAP apparatus and yet still maintain accurate monitoring of, and control over, CPAP treatment pressure.
A similar consideration applies for ventilators or apparatus for assisted ventilation that provide breathable gas to a patient at a controlled pressure. The gas is delivered to the patient, in the case of a ventilator, by way of a mask or an endotracheal tube. Patients with lung disease, neuromuscular disease, chest wall disease, or abnormalities of respiratory control may require ventilatory assistance. This is because they have various combinations of elevated airway resistance, stiff lungs and chest wall, ineffective respiratory muscles, or insufficient neural activation of the respiratory muscles. The need for ventilatory assistance is particularly common during sleep. Pressure controlled, time triggered ventilators, for example, deliver a relatively high inspiratory pressure (IPAP) for a fixed period of time (TI), and a relatively low expiratory pressure (EPAP) for another fixed period of time (TE). The cycle is then repeated indefinitely.
Pressure transducers typically are factory calibrated before delivery, to establish a zero pressure value (with respect to CPAP treatment pressure that is relative to atmospheric pressure) in terms of the transducer's offset or bias. The "zero offset value" thus corresponds to atmospheric pressure. Even so, due to the inherent variations in the transduced pressure, and due to aging of the transducer and its temperature dependency, the preset offset value can vary by the equivalent of .+-.1 cm H.sub.2 O leading to measurement error. This means that the patient must periodically return the CPAP apparatus to the manufacturer or servicer for re-calibration, else perform a re-calibration procedure themself, possibly requiring venting of the transducer to atmospheric pressure. It is therefore another preferred object of the invention to provide for auto-calibration of pressure transducer offset.
For convenience any reference to a "mask" hereafter is to be understood as including nasal, oral or face masks, and nasal prongs.